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(#) Insecure `HostnameVerifier`

!!! WARNING: Insecure `HostnameVerifier`
   This is a warning.

Id
:   `AllowAllHostnameVerifier`
Summary
:   Insecure `HostnameVerifier`
Severity
:   Warning
Category
:   Security
Platform
:   Android
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   1.5.0 (November 2015)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://goo.gle/AllowAllHostnameVerifier
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/AllowAllHostnameVerifierDetector.java)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/AllowAllHostnameVerifierDetectorTest.java)

This check looks for use of HostnameVerifier implementations whose
`verify` method always returns true (thus trusting any hostname) which
could result in insecure network traffic caused by trusting arbitrary
hostnames in TLS/SSL certificates presented by peers.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/InsecureHostnameVerifier.java:22:Warning: Using the
AllowAllHostnameVerifier HostnameVerifier is unsafe because it always
returns true, which could cause insecure network traffic due to trusting
TLS/SSL server certificates for wrong hostnames
[AllowAllHostnameVerifier]
    connection.setHostnameVerifier(new AllowAllHostnameVerifier());
                                   ------------------------------
src/test/pkg/InsecureHostnameVerifier.java:23:Warning: Using the
ALLOW_ALL_HOSTNAME_VERIFIER HostnameVerifier is unsafe because it always
returns true, which could cause insecure network traffic due to trusting
TLS/SSL server certificates for wrong hostnames
[AllowAllHostnameVerifier]
    connection.setHostnameVerifier(SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER);
                                   --------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/InsecureHostnameVerifier.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import android.content.Intent;

import java.io.IOException;
import java.net.URL;
import javax.net.ssl.HostnameVerifier;
import javax.net.ssl.HttpsURLConnection;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSession;
import javax.net.ssl.TrustManager;
import javax.net.ssl.X509TrustManager;

import org.apache.http.conn.ssl.SSLSocketFactory;
import org.apache.http.conn.ssl.AllowAllHostnameVerifier;

public class InsecureHostnameVerifier {
    protected void onHandleIntent(Intent intent) {
        try {
            URL url = new URL("https://www.google.com");
            HttpsURLConnection connection = (HttpsURLConnection) url.openConnection();
            connection.setHostnameVerifier(new AllowAllHostnameVerifier());
            connection.setHostnameVerifier(SSLSocketFactory.ALLOW_ALL_HOSTNAME_VERIFIER);
        } catch (IOException e) {
            System.out.println(e.getStackTrace());
        }
    }
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/AllowAllHostnameVerifierDetectorTest.java)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `AllowAllHostnameVerifierDetector.testBroken`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("AllowAllHostnameVerifier")
  fun method() {
     setHostnameVerifier(...)
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("AllowAllHostnameVerifier")
  void method() {
     setHostnameVerifier(...);
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection AllowAllHostnameVerifier
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="AllowAllHostnameVerifier" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'AllowAllHostnameVerifier'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore AllowAllHostnameVerifier ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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